This invention relates to dental impression materials.
Impression materials used in dentistry are one of several types of elastomers, such as polysulfides, condensation silicones, polyethers or polyvinyl siloxanes (addition-curable silicones). These materials are normally two-paste systems that are mixed immediately before use, then placed in contact with the tooth structure. Mixing of the two pastes initiates a chemical reaction that results in the formation of the elastic rubber impression material after setting, thereby forming a negative impression of the tooth structure involved. The addition-curable silicones, which exhibit fast curing speeds and low shrinkages, typically use a platinum-containing hydrosilation catalyst. This type of catalyst necessitates the use of silicon-containing oligomers, which are generally expensive. Also, the platinum-based catalyst can be inactivated by sulfur-containing impurities present in the latex gloves ordinarily used by dentists, as well as by certain medicaments used in the oral cavity. In addition, there may be undesirable hydrogen evolution from the decomposition of the hydrosiloxane cross-linkers that are present in the systems. This may increase the time and effort necessary to take an impression with these materials because extra precautionary steps have to be taken.
In view of these drawbacks, there is a need for dental impression materials that do not exhibit the various sensitivity problems described above.
The present invention provides a composition for use as a dental impression material. The composition comprises a polymerizable telechelic oligomer or polymer curable by ring-opening metathesis polymerization (ROMP), a filler system, and a ruthenium carbene complex catalyst, whereby the catalyst initiates the ring-opening metathesis polymerization of the composition. In an embodiment of the invention, the composition is a paste/paste system in which a base paste includes the polymerizable telechelic oligomer or polymer and a filler system and in which a catalyst paste includes the catalyst dissolved in an inert solvent and a filler system. In one example of the invention, the telechelic oligomer or polymer is a polydimethylsiloxane end-functionalized with norbornenyl groups and having between 5 and 5000 dimethylsiloxane units, and the catalyst is a ruthenium carbene complex with at least one of the ligands having a basicity higher than tricyclohexylphosphine. The composition of the present invention exhibits reduced sensitivity to sulfur impunities.
The present invention provides formulations for dental impression materials using a telechelic resin system that is cured by ROMP with the aid of a catalyst based on a ruthenium complex. The composition of the present invention comprises a catalyst paste and base paste in intimate admixture with one another in a paste/paste system. Curing is obtained by a ROMP reaction using a ruthenium carbene as a catalyst. Generally, in this system, the catalyst paste comprises a ruthenium catalyst for initiating polymerization, a solvent for the catalyst that is miscible or dispersible with the base paste, and fillers for optimizing viscosity for the application of the paste and for reinforcement of the cured material. Fillers for non-reinforcing purposes can always also be used. The base paste generally comprises a polymerizable oligomer and/or polymer resin system that is curable via the ROMP mechanism, and fillers as described above for the catalyst paste.
The type of oligomers and/or polymers that may be used in the present invention include telechelic (end functionalized) polymers with any of a variety of backbones, as long as the chain ends are functionalized with groups reactive by ROMP, such as norbornenyl groups. For example, the resin may be a telechelic polydimethylsiloxane terminated with 2-(5-norbornenyl)ethyl groups according the following structure: 
where n=5-5000, for example 27-1590. Other examples of telechelic polysiloxanes are those having the following structure: 
For an alternative example, the resin may be polytetrahydrofuran-polyethylene oxide copolymer terminated with norbornenyl groups. As yet another alternative example, the resin may be a norbornenyl carboxylate terminated polybutadiene.
The catalysts useful in the present invention include the ruthenium carbene complexes. The parent benzylidene ruthenium complex A, with the following structure, exhibits high air and water stability: 
The ring-opening metathesis activity of the parent complex A can be increased by substituting a saturated imidazole ligand for a tricyclohexylphosphine ligand. The ligands may be 4,5-dihydroimidazol-2-ylidenes, which have the following general structure: 
These substituted ligands have a basicity higher than that of tricyclohexylphosphine, as indicated by a higher pKa, which is believed to contribute to the higher activity. Ruthenium complex B, a derivative of complex A and having the structure shown below, is a substituted ruthenium carbene complex including such a ligand: 
Other derivatives of parent complex A can also be used in the resin system of the composition of the present invention, such as substituted ruthenium carbene complexes C and D having the following structures: 
The catalyst component of the dental impression material is formulated by dissolving the ruthenium carbene complex in an inert solvent. The solvent, or diluent, is chosen such that the solvent and the complex are miscible (soluble) or dispersible with the base pastes, and such that the solvent does not interfere with the reaction. The solvent may be, for example, 3-phenyl-heptamethyl-trisiloxane.
The fillers useful in the composition of the present invention include reinforcing and/or non-reinforcing (extending) fillers. Suitable reinforcing fillers include precipitated silicas, fumed silica, calcium silicate (Wollastonite), crystalline silica, and the like. The addition of reinforcing filler improves the mechanical strength, such as tensile and tear strengths, of the cured composition. Suitable non-reinforcing fillers include diatomaceous earth, aluminas, magnesias, titanium dioxide, zirconium silicate, calcium carbonate, metallic oxides, and the like. Surface-treated fillers may also be used. Typical surface treatments include silanization and the like. In accordance with the present invention, mixtures of fillers with different particle sizes may be used. A bimodal filler system blended with sub-micron ( less than 1 xcexcm) and micronsized particles (2-10 xcexcm) of close particle size distribution is used as a varying filler loading to provide dental impression materials with low, medium or high consistency, as defined by ISO Specification No. 4823 (2nd. Ed. 1992), suitable for use in all dental impression techniques. The filler may be present in amounts of from about 15 wt. % to about 50 wt. % of the composition. To adjust the consistency of the two-paste embodiment of the present invention to achieve either a low, medium or high consistency composition, the sub-micron sized filler and/or the micron sized filler may be adjusted in one or both of the catalyst and base pastes. The higher the consistency desired, the more beneficial it is to increase the sub-micron filler to a greater extent than the increase in the micron sized filler, whereby the sub-micron filler particles are worked into the interstitial spaces between micronized particles during mixing.